Digital communications systems often utilize a quadrature signal modulated as defined by a phase-point constellation to communicate digital information between transmission and reception components. The number of symbols or bits communicated per unit interval of time determines the order of a constellation and the number of phase points contained therein. That is, if N bits are communicated per unit interval of time, then the constellation has an order of 2.sup.N and contains 2.sup.N phase points.
The quadrature signal may be modulated using either phase shift keying (PSK) or amplitude and phase shift keying (APSK) modulation techniques. Since PSK modulation does not incorporate amplitude variations, intelligence is contained only within the phase of the quadrature signal. Using APSK modulation introduces amplitude as a component of signal intelligence. In other words, the information contained within the quadrature signal is a function of both the amplitude and the phase of the quadrature signal.
APSK constellations may be arranged in a rectangular (R-APSK) or polar (P-APSK) array of phase points. A given 16R-APSK constellation, for example, may have phase points located in a 4.times.4 grid. The 16-R-APSK constellation is well-known by the name 16-QAM. A given 16-P-APSK, non-QAM, constellation, by contrast, may have phase points in a two-ring, 8/8 equispaced phase-point pattern.
The 2.sup.N phase points in a PSK or APSK constellation are functions of the N bits in the digital data per unit interval of time. These bits may be uncoded or encoded. The use of encoded bits is usually preferred as the coding gain produces a significant reduction in the required signal-to-noise ratio of the quadrature input signal. The well-known "Ungerbach" or fully-encoded encoding scheme is often used for this purpose.
However, fully-encoded encoding requires different codes for different orders and types of modulation. This precludes the use of a functional "universal" demodulator. For this reason, pragmatic encoding is desirable. In pragmatic encoding, one or two of the N bits are encoded with the remaining bits per unit interval of time being transmitted uncoded. The well-known K=7, rate 1/2 convolutional encoding scheme is often used for encoding the one or two encoded bits.
In R-APSK constellations, the modulation along one axis is independent of the modulation along the other axis. That is, each symbol is modulated along either the I axis or the Q axis, but not both. For example, in the above-mentioned 16-R-APSK (i.e., 16-QAM) constellation, two of the four bits control modulation on the I axis and the other two bits control modulation on the Q axis. Since the I and Q axes are orthogonal, each pair of bits has no effect over the other pair. Such a constellation, therefore, is produced through independent bit modulation. Such constellations are moderately successful in the use of pragmatic encoding to increase efficiency.
However, in P-APSK constellations, the modulation along one axis is dependent upon the modulation along the other axis. That is, each bit is modulated along both the I and Q axes. For example, in the above-mentioned 16-P-ASPK constellation, each of the four bits influences modulation on both the I and Q axes. Such a constellation therefore is produced through dependent bit modulation. Using prior-art methodology, a dependent-modulation constellation is complex and presents a significant demodulation difficulty. Additionally, such constellations have not conventionally lent themselves to pragmatic encoding.
With a typical pragmatic encoding scheme having a modulation order of sixteen or more, the output of the encoding circuit is typically two encoded bits per unit interval of time. These encoded bits are passed to a phase mapping circuit, along with N-2 uncoded bits, to produce the quadrature signal to be transmitted.
In the course of transmission, propagation, and reception, the quadrature signal is often corrupted. If this corruption is an instantaneous event or "hit," then at the moment of corruption all bits would be corrupted, possibly beyond recognition.
The simultaneous corruption of all bits by a hit reduces the likelihood of producing a valid estimation of the corrupted bits when encoding is present. This effect may be reduced by interleaving bits relative to each other so as to prevent the simultaneous corruption of all bits by a hit. This is exemplified in U.S. Pat. No. 5,633,881, "Trellis Encoder and Decoder Based Upon Punctured Rate 1/2 Convolutional Codes," Zahavi et al., hereinafter referred to as "Zahavi." In Zahavi, a fully-encoded PSK modulation and demodulation methodology is taught. An input signal is fully encoded and processed into sets of P bits. The bits in each P-bit set are then interleaved, modulated into a 2P-PSK constellation, and transmitted. In the process of interleaving, each bit is delayed a successive integer number of delay units from zero to P-1, where a delay unit is the amount of delay required to temporally separate the bits in transmission. In this approach, the first bit is delayed zero delay units (i.e., is undelayed), the second bit is delayed one delay unit, and so forth to the last bit, which is delayed P-1 delay units. This methodology causes each bit in the P-bit set to be transmitted during a different delay unit of time, significantly reducing the effect of a hit.
On the receiver side, Zahavi teaches that the quadrature signal is demodulated into a composite hard-decision symbol. This composite hard-decision symbol is then deinterleaved into a set of P bits by delaying it a successive integer number of delay units from P-1 to zero to produce P deinterleaved hard-decision estimates. These P deinterleaved hard-decision estimates are then independently converted into P soft-decision estimates, corresponding to the P-bit set described above. The sets of P-bit estimates are then converted into N bit estimates and decoded into output data corresponding to the original input data.
The methodology taught in Zahavi is sufficient to provide temporal displacement for fully-coded bits. It does, however, suffer in that for large-order constellations, some bits would be delayed a significant number of delay units. For example, in a 64-APSK constellation, the last bit would be delayed five delay units. This large time span significantly increases the possibility that a hit may occur for a given symbol set.
The Zahavi methodology is also limited to use in connection with constellations using independent modulation, e.g., R-APSK constellations. With such constellations, each bit affects either the I component or the Q component of the quadrature signal, but not both. This allows a simple methodology wherein one of the I and Q components is delayed prior to transmission and the other of the I and Q components is delayed after reception.
Unfortunately, this Zahavi methodology is not feasible with constellations using dependent modulation, e.g., P-APSK constellations, as each encoded bit affects both the I and Q components of the quadrature symbol. Hence, a delay of one of the I and Q components affects both encoded bits and fails to produce the requisite temporal offset. For this and other reasons, P-APSK constellations are usually considered undesirable even though they constitute a more efficient usage of the constellation space.
It is therefore a problem of the prior art that constellations utilizing dependent modulation are not readily realizable in optimal configurations.
It is an additional problem of the prior art that pragmatic encoding is impracticable for constellations utilizing dependent modulations.
It is also a problem that practicable temporal-displacement techniques are impracticable for constellations utilizing dependent modulation.
Furthermore, it is a problem of the prior art that efficient large-order constellation demodulators utilizing temporal displacement are not provided.
It is again a problem of the prior art that no universal pragmatic-encoding demodulator is practicable.